1. Field of the Invention
The present invention relates to a recording ink which includes pigment particles as a coloring agent, which are dispersed in a solvent containing at least water.
2. Discussion of Related Art
Conventionally, an aqueous dye ink composition is widely used as a recording ink for use in an ink jet printing method. This ink composition includes various kinds of water-soluble dyes, which are dissolved in water or a mixture of water and a water-soluble solvent, and various additives as needed, as disclosed in JP-A-63-51485, JP-A-63-56575, and JP-A-1-198671, for example. However, the aqueous dye ink composition, when actually used for printing images on a recording medium, exhibits insufficient water-fastness and light-fastness of the printed images, thus leaving some room for improvements.
In view of the above, there has been developed in recent years a pigment ink composition which contains a pigment or pigments, such as carbon black or Diazo Yellow, in place of the dyes as described above, and thus exhibits improved water-fastness and light-fastness of printed images. The pigment ink composition may be suitably used in an ink jet printing method, as disclosed in JP-A-57-10660, JP-A-57-10661, U.S. Pat. Nos. 4,597,794, 5,160,370, 5,229,786 and JP-A-2-276876, for example. The pigment ink exhibits higher water-fastness than the dye ink, since the pigment sticks or adheres to a recording medium, such as paper, due to the use of resin in the ink composition. Further, pigments are less sensitive to light than dyes, and, therefore, the pigment ink exhibits higher light-fastness than the dye ink.
However, the pigment ink is likely to undergo chronological changes, resulting in reduced stability, since the ink contains pigment particles which are not dissolved in an ink solvent. To solve this problem, JP-A-5-202324 has proposed to use a non-aqueous solvent, and control the average particle size of the pigment to the range of 1.0 .mu.m to 2.0 .mu.m, so as to improve the chronological stability of the ink, and print quality of recorded images. The print quality may be evaluated in terms of blurring and print density, for example.
Generally, the viscosity of the ink used for the ink jet printing is controlled to a desirable range of several centipoises. When the pigment having such viscosity have an average particle size in the above range of 1.0 to 2.0 .mu.m, however, the pigment particles have increased sedimentation velocity, according to Stokes' law, resulting in reduced chronological stability of the ink. Although the stability may be improved by increasing an amount of a dispersing agent added to the ink composition, this in turn increases the viscosity of the ink, and thus requires high energy input in order to eject the ink from a nozzle of a printhead of an ink jet printer. This eventually pushes up the costs of a power source for supplying the ink jetting energy or power, and a driving circuit for controlling the energy.